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Diablo 3 has almost completely overhauled the character development as shown in the previous editions. Most importantly, the manual method for allocating attribute points and limited skill is no longer valid. In the earlier editions of the game, the character builds were very important with focus on the right selection of some attributes as against level and skill choices. In Diablo 3, all characters are almost save except for the gear the hero chooses for them. The sill and attribute systems have been changed a bit too.
The changes to skill management are vital to understanding the new strategies required for controlling Diablo 3. Unlike earlier editions in which Mana was the main resource for generating special abilities, Diablo 3 has specific resources for each class. All these resources operate in different way. For example, you have fury for the Barbarians, and fury increases according to damage. Then, the Wizards use arcane power that recharges way faster than the Mana but can be obtained only of a fixed quantity. There is hatred and discipline for the Demon Hunters, while the monks have spirit which grows slowly. On the other hand, Witch Doctors, have Mana like before which increases at every level but recharges slowly as compared to the other resources.
Therefore, skills are way more limited in their cost and effect in Diablo 3. The potion-quaffing has given way to another interesting method in classes like the Demon Hunter.
Additional pace comes from XP bonuses for kill streaks for the Barbarian. Then, there are the health globes. They are the main source of health and bringing back a fixed percentage of health both for the hero and any party members that are nearby. Monsters and bosses drop the health globes. It's one of the key improvements in Diablo 3.
Also, in Diablo 3, the Town Portals do not exist any longer. The Stone of Recall, which is won in the beginning quest, works like the Stone of Recall in World of Warcraft. It takes you back to town. But the impact of this is that you can not run back to town when up against a tough enemy. You can run indefinitely in Diablo 3 without worrying about stamina. The movement speed is much more fixed and you can not run away from enemies.